In conventional planning of radiation treatments, a forward planning approach is commonly employed, in which the treatment planner manually alters treatment parameters until an acceptable dose distribution is obtained. For Intensity Modulated Radiation Therapy (IMRT), an inverse planning approach is usually employed where the treatment planner specifies requirements on the dose distribution, which are taken as input to an optimization algorithm trying to find the set of treatment parameters which most effectively produces the desired dose distributions.
The general procedures of inverse treatment planning, and the various steps involved when using a Treatment Planning System (TPS) for optimizing a treatment plan, are well-known to a person skilled in the art of radiotherapy treatment planning and details thereof are therefore not further described here.
When prescribing a radiation dose to a target, e.g. a tumor, a homogeneous dose is usually desired. However, under certain circumstances it would be advantageous to deliver a deliberately heterogeneous dose to a target volume or another Region Of Interest (ROI) such as an Organ At Risk (OAR). This would be desirable for example in the field of adaptive radiotherapy where a treatment plan is re-optimized during the course of treatment. Radiotherapy treatment is usually fractionated, i.e. the treatment time is extended, often over several weeks, where fractions of the total planned dose are administered daily. If the delivered dose of some fractions for any reason does not match the intended dose, some sub-regions of a ROI might become underdosed (having “cold spots”) or overdosed (having “hot spots”). In this context, adaptive radiotherapy refers to the process of modifying a treatment plan in between fractions, in order to compensate for a delivered dose which deviates from the intended dose. Thus, when adapting the treatment plan, a deliberately inhomogeneous dose prescription, compensating for cold and/or hot spots, might be used as input to the treatment planning system.
Another case where prescribing a deliberately inhomogeneous dose is advantageous is when using functional imaging information, for example obtained from a PET-scan, as input to the TPS. Such functional imaging information could indicate regions within a target which are more or less radiosensitive, thus indicating that different doses should be delivered to different parts of the target.
Regardless if a homogeneous or a heterogeneous dose is prescribed to a target volume, a clinically useful treatment plan must take position uncertainties and organ motion into account. In order to do this, margins are often applied around a region of interest, such as a target volume, to ensure that the whole volume receives the intended dose. Such a margin when applied to a Clinical Target Volume (CTV) defines the Planning Target Volume (PTV). In order to limit the dose to healthy tissue as much as possible, the margin should not be larger than necessary. When planning delivery of a heterogeneous dose to a region, a margin around the region will not help to achieve a plan which is robust with respect to the different dose levels within the region.
Other methods for obtaining robust treatment plans, which do not depend on the use of margins but use a probabilistic approach, are also known in the art. Such methods often involve consideration of a number of more or less probable scenarios, for example defined by different shifts of the CTV. This kind of robust treatment planning is time-consuming and computationally intensive, since many different scenarios must be analyzed. Furthermore, such methods would be of limited use when planning delivery of a heterogeneous dose due to the large number of scenarios that would have to be taken into consideration.
The present invention aims at mitigating these drawbacks and achieving treatment planning of heterogeneous doses which is both computationally efficient and robust, i.e., insensitive to position uncertainties and organ movements.
Prescribing a heterogeneous dose implicates the use of a voxel-specific dose objective for the optimization of a treatment plan. As used herein, a voxel-specific dose objective is a dose objective for a volume comprising more than one voxel (volume element of the treatment volume), where the voxels of the volume have specific and possibly different dose objectives. In this regard, a prescribed heterogeneous dose corresponds to a voxel-specific dose objective defining different dose objective dose values for different voxels.